How Safe Is Our Water?

As a child growing up in rural China, Chuanwu Xi used to drink water directly from a stream near his home. It’s a pleasure he says
he wouldn’t risk today, because surface contamination has rendered the stream unsafe—but
the memory may explain his abiding interest in the quality of the water we drink.

In the United States, where he has lived since 2001, water-treatment plants do an
excellent job of purifying our drinking water, says Xi, an assistant professor of
environmental health sciences, but once that water leaves the treatment plant, things
change. Inside our drinking-water distribution systems, millions of bacteria collect
on the surfaces of pipes and other plumbing materials and grow into microscopic communities
called biofilms, which fortify bacterial resistance to disinfectants and can be a
reservoir for some opportunistic pathogens. While they pose little or no threat to
healthy adults—in fact, Xi himself prefers drinking tap water to bottled H2O—biofilms
in the drinking-water distribution system can have adverse health effects on immuno-compromised
individuals, including infants, transplant recipients, persons with HIV, and the elderly.

In the past three decades, officials have registered several hundred outbreaks of
waterborne disease in the U.S. due to contaminated drinking water.

To help address the problem, Xi is collaborating with Lutgarde Raskin, professor of
civil and environmental engineering at the University of Michigan, to examine the
molecular mechanisms that underlie bacterial resistance to disinfectants in drinking-water
distribution systems. Using DNA microarray technology and flow cytometry, the scientists
are sorting the cells of two bacterial strains, E. coli and Mycobacterium avium, from
biofilms and analyzing the genome-wide gene expression of each strain in biofilms
and/or in response to disinfectant. E. coli is of interest because it’s a model organism,
Xi says, and M. avium is an opportunistic pathogen. He and his research team hope
to identify those genes in each bacterium that respond to disinfectant as well as
those genes that are involved in the formation of biofilms.

If they can determine the molecular mechanisms of resistance, they may be able to
develop new disinfection approaches that would circumvent the mechanisms and help
cleanse drinking water of potentially harmful biofilms. Although they’d like to find
a way to stop the growth of biofilms altogether, Xi says it’s not likely they’ll succeed,
“because bacteria are ‘cleverer’ than us.” He is optimistic, though, that he can identify
ways to reduce the extent of biofilms and therefore reduce their threat to human health.

Photo by Peter Smith.

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